Support application subscriptions (see the :ddocs:`official docs <monetization/overview>` for more info).
- New types: :class:`SKU`, :class:`Entitlement`.
- New :attr:`Interaction.entitlements` attribute, and :meth:`InteractionResponse.require_premium` response type.
- New events: :func:`on_entitlement_create`, :func:`on_entitlement_update`, :func:`on_entitlement_delete`.
- New :class:`Client` methods: :meth:`~Client.skus`, :meth:`~Client.entitlements`, :meth:`~Client.create_entitlement`.
